It will depend greatly on the plasma you have because they all have different input ports.

Best quality is with component video, your DVD will be able you output that... maybe Xbox.

RGB over scart is good quality and will allow you to use cheap scart switcher. You can then connect 3-4 devices to the switcher and just a single cable to the Plasma. The switcher can automatically select the active device.

I would recommend you to use component video for the DVD to squeeze out maximum quality (look for progressive scan mode) and RGB scart for all other devices to minimise on calbles

VCR usually don't have RGB output, you will have to use s-video or composite (one yellow phono connector). Older plasmas don't support s-video or composite.